Agricultural combines are large self-propelled vehicles used for harvesting, threshing separating and cleaning agricultural grain crops in the field as they are being harvested. A harvesting head is typically disposed across the front of the combine vehicle and is supported on a feederhouse. This harvesting head is configured to cut and convey the crop into the combine vehicle itself where it is threshed, separated, and cleaned.
The feederhouses are raised and lowered by hydraulic cylinders that are coupled between the feederhouses and the combine vehicle itself. As the hydraulic cylinders are extended, the feederhouse pivots upward with respect to the combine vehicle and the harvesting head mounted on the feederhouse is raised higher above the ground. As the hydraulic cylinders are retracted, the feederhouse pivots downward with respect to the combine vehicle and the harvesting head mounted on the feederhouse is lowered closer to the ground.
The feederhouse is constantly raised and lowered as the feederhouse travels through the field in order to maintain the feederhouse at a constant height. With increased combine speed through the agricultural field and the greater weight and size of modern harvesting heads, it has been difficult to provide hydraulic circuits to drive the feederhouse cylinders that can respond quickly to changes in the contours of the ground. Closed center hydraulic circuits have been disfavored because they are slow to respond. Open center hydraulic circuits have been increasingly favored.
Unfortunately, however, open center hydraulic circuits require more energy to operate due to their continuous flow of hydraulic fluid under pressure even when the hydraulic cylinders are not being filled or emptied. This equates to increased fuel consumption, increased load on the combine engine and increased pollution.
What is needed is a hydraulic circuit for raising and lowering harvesting heads that has lower power demands, fuel consumption and responds faster to command signals. It is an object of this invention to provide such a hydraulic circuit.